In recent years, in office automation (OA) equipment, such as word processors, personal computers, and facsimile machines, various measuring instruments, such as medical measuring instruments, and other devices, ink jet printers have been extensively used for printing information at a high density. As is well known in the art, in the ink jet printers, ink droplets are ejected from head sections of the printers and are deposited directly onto a recording medium, such as recording paper, to perform monochrome or color printing. The ink jet printers have many advantages such as printing can be performed on even a three-dimensional recording medium, running cost is low since plain paper can be used as the recording medium, the head can be simply loaded, the step of transfer, fixation and the like is unnecessary, color printing is easily performed, and a sharp color printed image can be provided. The ink jet head can be classified into several types according to the method for ejecting ink droplets from the head. For example, in an ink jet head of a piezoelectric system, a piezoelectric element is used as pressurizing means. In this case, electrostrictive effect provided by this piezoelectric element is utilized to create a pressure wave within an ink chamber, filled with an ink, in the head section, permitting the ink to be ejected through a nozzle in the head section. In an ink jet head of a bubble jet system, a heating element is used as the pressurizing means, and the heating element is heated to form a bubble, permitting an ink to be ejected through a nozzle in the head section. Further, an ink jet head of a static electricity-driven ejection system is also known wherein ink droplets are ejected by utilizing static electricity. The ink jet head according to the present invention can be advantageously applied to ink jet heads of these and other systems.
The conventional ink jet head generally comprises: a plurality of ink chambers which are disposed at equidistant spaces and function as ink flow passages and pressurizing chambers for ejecting ink; and a nozzle plate mounted on the front end of the ink chambers and equipped with nozzles, for ejecting an ink, corresponding to the ink chambers; and pressurizing means for pressurizing ink within the ink chamber in response to the demand for printing. The pressurizing means has a driving element for creating a driving force for pressurizing the ink chamber. The driving element is a piezoelectric element in one case and a heating element in another case.
The structure of the ink jet head will be described in more detail. For example, as can be understood from FIG. 1 showing an exploded view of the ink jet head, an ink jet head 10 of the piezoelectric system comprises several members. An ink chamber member 11 has a plurality of ink chambers 12 which serve as an ink flow passage and as a pressurizing chamber for ejecting an ink. A nozzle plate 13 is mounted on the front end of the ink chamber member 11 and is equipped with nozzles 14, for ejecting an ink, corresponding to the ink chambers 12. As described above, the ink pressurized within the ink chamber 12 can be ejected as droplets through the bore of the nozzle 14. In the ink chamber member 11 shown in the drawing, pressurizing means is mounted on the open face of the ink chamber 12. In the example shown in the drawing, the pressurizing means comprises a diaphragm 15 for creating a change in volume of the ink chamber 12, a piezoelectric element 17 as a driving element for distorting the diaphragm 15, and a base 18 for fixing the piezoelectric element 17.
The ink chamber member 11 has a plurality of ink chambers 12, in a deep groove form, serving both as ink flow passages and as pressurizing chambers for ejecting an ink, and the ink chambers 12 correspond respectively to nozzles 14 formed in the nozzle plate 13 and are designed so that a corresponding nozzle is disposed in one ink chamber. The ink chambers 12 are disposed at equidistant spaces and parallel to one another using a partition for separating adjacent ink chambers from each other. In this case, in order to enhance the resolution of the ink jet head, the spacing between the ink chambers 12 formed in the ink chamber member 11 should be narrowed. The ink chamber member 11 can be generally joined to the nozzle plate 13 with the aid of an adhesive.
The diaphragm 15 is a component characteristic of an ink jet head 10 of a piezoelectric system. When the piezoelectric element 17 is stretched by the electrostrictive effect, the diaphragm 15 is flexed to create a change in volume within the ink chamber 12. A reduction in the volume within the ink chamber 12 causes the ink filled into the chamber to be pressurized, and a part of the ink is ejected as droplets successively through the nozzle 14. The diaphragm 15 generally comprises a sheet having a small thickness of about 3 to 5 .mu.m and, provided on one side thereof, islands 16 each comprising a protrusion having a height of about 20 .mu.m. Upon stretching of the piezoelectric element 17 by the electrodistrictive effect, the islands 16 function to surely transmit the distortion created by the stretching to the ink chamber 12. For this reason, the islands 16 are disposed so as to form a laminate together with the respective corresponding ink chambers 12 and the corresponding portions of the piezoelectric element 17. The ink chamber member 11 and the diaphragm 15 can also be joined to each other with the aid of an adhesive.
The piezoelectric elements 17 correspond respectively to the ink chambers 12 in the ink chamber member 11 and are separated from one another in order to avoid influencing other ink chambers 12. The piezoelectric elements 17 separated from one another are fixed onto a base 18. In general, the piezoelectric elements 17 separated from one another are prepared by joining piezoelectric elements, which are not initially in a separated state, to a base with the aid of an adhesive and then selectively separating the piezoelectric elements alone from one another by cutting. After the formation of an integral assembly of the piezoelectric elements and the base, the piezoelectric elements may be joined respectively to corresponding islands, formed on the diaphragm, with the aid of an adhesive.
In the above and other ink jet heads, the performance of the ink chambers serving both as ink flow passages and as pressurizing chambers for ejecting ink directly influences printing properties and hence is very important. At the outset, an ink chamber member constituting the ink chambers will be described. The ink chamber member in the conventional piezoelectric ink jet head has been generally produced by injection-molding an organic material, for example, "Epox" (tradename of an epoxy resin). The ink chamber member constituted by the organic material, however, has poor rigidity, posing problems including that satisfactory pressure cannot be applied to the ink at the time of pressurizing.
Another method for producing the ink chamber member is such that a powder of an oxide, such as ZrO.sub.2, is used instead of the organic material and the powder is molded by powder injection molding into an ink chamber member. In this method, however, use of a mold is necessary, and a very large pressure should be applied at the time of filling of the mold with the powder as the raw material, making it difficult to use a mold having a structure fine enough to form fine ink chambers.
Etching also may be mentioned as a method suitable for the formation of fine ink chambers. For example, according to this method, a groove pattern can be finely formed on the surface of a metallic sheet having a thickness of about several hundred .mu.m. In this method however, regarding an increase in density, the minimum possible width of the groove is approximately equal to the sheet thickness, and, hence, the above method cannot be said to be satisfactorily effective. Further, according to the etching method, the metallic sheet can be also etched off during formation of the groove, and therefore, if use of the metallic sheet as the ink chamber member is contemplated, one side of the metallic sheet should be clad with an additional member for preventing etching off of the sheet, rendering the production process complicated.
A further method for forming the ink chamber member is photolithography, using a photosensitive resin generally called "photoresist" or "resist," which is known from and disclosed in, for example, Japanese Examined Patent Publication (Kokoku) Nos. 62-59672 and 2-42670. This method comprises the steps of: applying a resist onto the whole surface of a substrate on which ink chambers are to be formed; selectively exposing the resist film to suitable light according to a pattern of the ink chamber to be formed; and dissolving and removing areas, not insolubilized by the exposure, by using a developing solution to prepare a substrate provided with desired ink chambers of the pattern of the cured resist. Photolithography uses a resist widely adopted in the production of semiconductor devices such as LSIs and VLSIs.
FIGS. 2(A) and (B) are cross-sectional views showing, in order, the steps for forming ink chambers by the conventional photolithography. As shown in FIG. 2(A), a resist is coated onto the surface of a substrate 31 to form a resist layer 32 which is then subjected to pattern exposure through a photomask 33. Since the resist used herein is a negative-working resist sensitive to ultraviolet radiation, the photomask 33 is made of glass and constructed so that an ultraviolet radiation is passed through the glass in its areas corresponding to partitions of the ink chambers and a chromium layer is applied to the other areas in order to prevent the passage of the ultraviolet radiation. Light for exposure indicated by an arrow is ultraviolet radiation from a light source (not shown).
As a result of the pattern exposure, the exposed areas in the resist layer 32 are rendered insoluble in a developing solution. The exposed resist layer 32 is developed with a suitable developing solution to dissolve and remove unexposed areas (soluble areas). As shown in FIG. 2(B), a cured resist pattern 32 corresponding to the form of desired ink chambers is provided. The residual resist pattern 32 serves as a partitioning member for separating adjacent ink chambers from each other. The substrate 31 serves as a base plate member. In the photolithography shown in the drawing, a negative-working resist was used as the resist. Use of a positive-working resist, wherein the exposed areas are solubilized and removed by dissolution, instead of the negative-working resist has also been reported.
Ink chambers in an ink jet head of another system, a bubble jet head, can be produced in the same manner as described in connection with the production of the piezoelectric ink jet head. Specifically, ink chambers and nozzles are basically common to head sections in these two systems. In the bubble jet system, however, the piezoelectric element and the diaphragm are not used, and, instead, heating elements and related members disposed so as to correspond respectively to the ink chambers are provided on a highly rigid substrate.
The conventional ink jet heads described above cannot cope with high-density printing which has been desired particularly in recent years.
In recent years, high-density printing of not less than 180 in terms of dots per inch (dpi) is required in the field of printers. In this connection, it is needless to say that, in the ink jet head, the distance between ink chambers and therefore the distance between nozzles should be a small value equivalent to at least 180 dpi. The "spacing equivalent to 180 dpi," when specifically expressed in terms of length, means that the ink chambers and the nozzles are formed at spacings of 141 .mu.m (see spacing d between adjacent ink chambers 12 and spacing d between adjacent nozzles 14 in FIG. 1). Specifically, in the ink chamber member, an ink chamber and a partitioning member for separating ink chambers should be formed in a limited length of 141 .mu.m. For example, when the ratio of the width of the ink chamber to the thickness of the partitioning member is 1:1, the width of the ink chamber and the thickness of the partitioning member are respectively 70.5 .mu.m and 70.5 .mu.m. Thus, the width of the ink chamber decreases with increasing the printing density. In this connection, however, it should be noted that, although high-density printing can be achieved by decreasing the width of the ink chamber, excessively small ink dots printed on a recording medium do not offer good print quality. Ensuring a large dot ejected, in other words, ejection of a satisfactory amount of an ink through each nozzle, is necessary for avoiding the deterioration of the print quality. A larger volume of the ink chamber is better for meeting this requirement. This requires the provision of an ink chamber of which the height of the partitioning member is large.
Returning to the formation of the ink chamber member, in the case of injection molding, which is the most commonly used conventional method, the formation of a fine structure equivalent to a printing density of about 120 dpi is the limit of the injection molding because the formation of a fine structure leads to a fear of the mold per se being broken and, in addition, molding cannot be performed with high accuracy.
On the other hand, in the case of the etching method, it is easy to finely fabricate a flat material, and, hence, this method can satisfactorily cope with a patterning of about 180 dpi. In the conventional etching method, however, the possible patterning width is influenced by the thickness of the member to be etched. For example, when the formation of an ink chamber member having a fine structure equivalent to 180 dpi, that is, an ink chamber width of 70.5 .mu.m and a partitioning member thickness of 70.5 .mu.m, is contemplated, patterning cannot be carried out unless the thickness of the member to be etched is 70 .mu.m or less. This means that, when higher density is desired, the height of the ink chamber cannot be increased.
This is true of the formation of ink chambers by photolithography using a resist. Specifically, when the thickness of the resist is 50 .mu.m or more, patterning of the resist layer by the conventional photolithography is impossible. The reason for this is as follows. A photosensitive resin commonly used as a resist is designed on the premise that the thickness is 50 .mu.m or less. In fact, when the photosensitive resin is used in a thickness exceeding 50 .mu.m in the resist process, problems such as underexposure and underdevelopment occurs, making it impossible to achieve fine patterning.
When the resist layer is patterned by the conventional photolithography, the upper limit of the possible patterning width:resist layer thickness ratio (aspect ratio: in the ink chamber member, as will be described below, the aspect ratio being defined by the ink chamber width:ink chamber height ratio) is about 1:2. When the aspect ratio exceeds 1:2, problems occurs including that the resist pattern as the partitioning member of the ink chamber is deformed, or otherwise the base portion close to the substrate is narrowed, resulting in the creation of the so-called "reverse taper" form, or otherwise the pattern can no longer stand up straight and is united with the adjacent pattern.
Regarding the above defect of the resist pattern, reference may be made of a scanning electron photomicrograph (magnification: 500.times.) shown in FIG. 5. The resist pattern in the photograph is one formed by the present inventors. Specifically, in the formation of the resist pattern, a negative-working thick layer resist (THB 30 (tradename) sensitive to an ultraviolet radiation, manufactured by Japan Synthetic Rubber Co., Ltd.) was provided, and patterning was carried out on an aluminum substrate so that the resist pattern width was 30 .mu.m, the resist pattern spacing was 30 .mu.m, and the resist pattern height was 50 .mu.m. Conditions applied were as follows.
Coating of resist: 1000 rpm.times.10 sec
Prebaking: 100.degree. C., 5 min
Exposure conditions: 100 mW/cm.sup.2, 35 sec
Post-baking: 100.degree. C., 15 min
As can be seen from the photograph shown in FIG. 5, when the aspect ratio of the resist pattern was 1: about 1.7, the formed pattern was satisfactory, that is, a defect-free resist pattern wherein cured partitions were orderly arranged.
Further, the formation of a resist pattern was repeated in the same manner as described above, except that the aspect ratio was gradually increased. As a result, a defect began to occur when the height of the resist pattern was around 60 .mu.m.
FIG. 3 illustrates an example where the resist pattern 32 formed on an aluminum substrate 31 is in a reverse taper form. As is apparent from the drawing, the resist pattern 32 is formed such that its side wall is not perpendicular to the substrate 31, and thus the width of the partition is narrow at the base 32a. This reverse tapered pattern is attributed to the fact that, at the time of exposure, a satisfactory amount of light does not reach the base 32a in contact with the substrate 31 and, consequently, the exposure in the thicknesswise direction of the resist layer 32 becomes heterogeneous. That is, since the thickness layer resist is a negative-working resist wherein unexposed areas are dissolved and removed by development, the portion, around the base 32a of the resist layer 32, which is less likely to be exposed is overdeveloped. This tendency becomes significant with increasing the thickness of the resist layer. When the resist pattern to be used as the partitioning member of the ink chamber member is in such a reverse taper form, the area, which participates in intimate contact between the substrate 31 and the resist pattern 32, is reduced and, in the worst case, the resist pattern 32 is separated from the substrate 31. Even though the resist pattern 32 is not separated from the substrate 31, since the wall thickness of the pattern decreases with an increase in the aspect ratio of the resist pattern 32, the resist pattern 32 often falls particularly in the thin portion as shown, for example, in FIG. 4 as a cross-sectional view. Deformation of the resist pattern in this way makes it impossible to use the resist pattern as partitioning members in the ink chambers.
FIG. 6 is a scanning electron photomicrograph (magnification: 350.times.) showing an example where a resist pattern is formed in a reverse taper form on an aluminum substrate and walls of the pattern in their apex are partially united with each other. The resist pattern shown in this photograph was formed substantially the same manner as described above in connection with FIG. 5, except that, for comparison, the width of the resist pattern, the spacing between walls of the pattern, and the height of the pattern were 50 .mu.m, 50 .mu.m, and 100 .mu.m, respectively, and the resist solution was coated twice to increase the layer thickness.
From the above results, it is apparent that, when the resists used above and other commercially available resists are used to form a resist pattern by the conventional method, the aspect ratio of the resist pattern should be limited to until about 1:2 in order to avoid the creation of various defects.